The definition of pressure is the force acting on the unit area, in which the magnitude of the force (Kgf) acting per 1 Cm2 is most commonly used, i.e. Kgf/Cm2. Pressure may also be expressed in terms of the corresponding height of the mercury (mmHg) and the height of the water column (mmHg).
Recently, the spread of the International Unit System (SI Unit System) is expanding for the purpose of international unit unification, in which the force acting per square meter, N/m2, the size of N (Newton), is expressed as Pa (Pascal).
In this case, Pa is a fairly small unit and often uses either Kpa (kilo-pascal) or Mpa (mega-pascal) that is 1,000 times larger.
Bar(bar) is used as the atmospheric pressure unit.

• Physics Theoretical : Absolute pressure based on full vacuum
• General engineering : use of instrument pressure based on atmospheric pressure
• Absolute pressure : pressure measured based on full vacuum
  Absolute pressure [kgf/Cm2abs] = Gauge pressure[kgf/Cm2] + Atmospheric pressure1.0331[kgf/Cm2]

For example, if air pressure of 1Kgf/Cm2G (≒2Kgf/Cm2abs) is compressed to a volume of 1/4 as shown, the pressure will be 7Kgf/Cm2G (≒8Kgf/Cm2abs), increasing by approximately 4 times the absolute pressure.
This relationship is called BOYLE's law.